Purged anode low effluent fuel cell

ABSTRACT

A hydrogen-fueled fuel cell reacts residual fuel in the exhaust of the anode flow field either in a catalytic converter or by feeding the anode exhaust into the cathode oxidant stream. Control of flow of anode exhaust into the cathode oxidant stream may be in response to a flammability sensor, a gas composition analyzer, current output, or periodically in response to a timer; the anode exhaust may be fed either upstream or downstream of the cathode air inlet blower.

TECHNICAL FIELD

This invention relates to dissipating residual fuel emanating from theanode flow field of a fuel cell, thereby permitting the flow to besufficient to purge inert residue which accumulates on the anode side ofthe cell.

BACKGROUND ART

In all fuel cells, particularly where the oxidant is supplied by air,inert gaseous molecules, particularly nitrogen, diffuse through theelectrolyte and accumulate on the fuel side (anode) of the cell. Theaccumulation of inert residue ultimately blocks the hydrogen fuel fromreaching the anode catalyst and the electrolyte, which ultimately leadsto significant loss of cell performance. Exhausting or venting ofeffluent from the anode, which invariably will contain residual fuel, isundesirable in most instances because unreacted fuel can pose a safetyhazard and will generally be perceived as polluting the atmosphere.Typical fuel cell power plants are designed for operation on hydrocarbonfuels (natural gas, methanol or gasoline) and usually utilize the anodeexhaust as a source of fuel for a burner required in the apparatus thatprocesses the fuel to make it a hydrogen-rich stream; all fuel in theanode exhaust is combusted, so that no unburned fuel will leave thepower plant.

Although some space and military applications have utilizedhydrogen-fueled fuel cell power plants, with no fuel processor, andeither dead ended the anode (not allowing any effluent therefrom) orsimply dumped the anode exhaust into the environment, there are otheruses for hydrogen-fueled power plants, particularly in vehicles such asautomobiles, which can neither tolerate the performance degradationwhich can be caused by buildup of inert residues on the anode side nortolerate a fuel-containing exhaust. A dead-ended fuel stream causestrace level impurities to accumulate, which requires a fuel-exit purgeinto the ambient.

DISCLOSURE OF INVENTION

Objects of the invention include provision of a hydrogen-fueled fuelcell which has sufficient flow in the anode flow field to purge theanode of inert residue, while providing fuel-free exhaust into theambient.

According to the present invention, a fuel cell anode exhaust iscombusted by catalytic reaction with the oxidant, typically air, beforebeing exhausted to ambient atmosphere. In accordance with a first formof the invention, the anode exhaust is fed to the cathode inlet manifoldso that the unreacted hydrogen can mix with the inlet air and safelyreact on the platinum catalyst on the cathode side of the fuel cell, toeliminate substantially all hydrogen from the emissions of the cathodeside of the cell. Reaction of the hydrogen on the cathode side formswater which has an additional benefit, in some fuel cells such as protonexchange membrane (PEM) fuel cells, of improving the water balance ofthe fuel cell, because it adds to the process water produced in thenormal electrochemical reaction of the cell. In this form of theinvention, the anode exhaust may be applied either ahead of or behindthe air blower, and the rate of flow of anode exhaust mixed with inletair can be controlled in response to the hydrogen content in the inletair, the gas analysis of the anode effluent, the current being suppliedby the fuel cell to a load, or other parameters. The exhaust flow fromthe anode can either be metered continuously, at a steady or varyingrate, or can be turned on and off thereby to provide periodic purgingaccording to a predetermined schedule. According to the invention inanother form, the anode effluent may be passed through a catalyticconverter (mechanically similar to those conventionally used in motorvehicles to convert nitrogen oxides, carbon monoxide and unbondedhydrocarbons) thereby to consume hydrogen before venting to atmosphere,such as in the exhaust of a fuel cell powered electric automobile.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1-5 are stylized, schematic diagrams of a portion of a fuel cellsystem in which the present invention is effected by passing the anodeexhaust through the cathode flow field.

FIG. 6 is a stylized, schematic diagram of a portion of a fuel cellsystem in which the present invention is effected by passing the anodeexhaust through a catalytic converter.

FIG. 7 is a plot of various fuel cell parameters as a function of time,with and without fuel being recirculated into the cathode stream.

BEST MODE FOR CARRYING OUT THE INVENTION

An exemplary PEM fuel cell in which the present invention may bepracticed is shown in U.S. Pat. No. 5,503,944. As is known, a fuel cellsystem or power plant is comprised of many fuel cells disposed adjacentto each other so as to form a cell stack assembly. Referring to the FIG.1, each component cell will include a membrane 8; an anode substrate 10,and an anode catalyst layer 12; a cathode substrate 18, and a cathodecatalyst layer 20; an anode flow field plate 2'; and a cathode flowfield plate 2. The flow field plates 2 and 2' are positionedback-to-back with the projections 4 and 4' being disposed inface-to-face contact. The grooves 6 and 6' combine to form coolant waterflow fields on the anode and cathode sides of the electrolyte membrane8. The projections 14' abut the anode substrate 10; and the projections14 abut the cathode substrate 18. The grooves 16' thus form the anodereactant flow field; and the grooves 16 form the cathode reactant flowfield.

FIG. 1 also shows, schematically, the system components of the cellstack assembly. All of the anode reactant flow fields 16' in the powerplant are supplied with a hydrogen-rich fuel gas reactant from a supplysource tank 22 thereof. The hydrogen reactant flows from the supplysource 22 to the anode flow fields 16' through a supply line 24. Theamount and pressure of hydrogen-rich fuel flowing through the supplyline 24 is controlled by a supply valve 26 and a supply regulator 28which may be manually or automatically operated. All of the cathode flowfields 16 are supplied with an oxidant, such as air, via an air blower30 and an air line 32. The oxygen used in the electrochemical reactionis thus derived from ambient air in this embodiment.

Coolant water is circulated through the power plant cell units via lines34. The coolant water passes through coolant passages 36 between theplates 2 and 2'. Coolant water is circulated by a pump 38, which can bea fixed or variable speed pump. The coolant water circulating loopincludes a heat exchanger 40 which lowers the temperature of the waterexiting from the coolant passages 36. A branch line 42 leads from theline 34 to a reservoir 44 that may be open to ambient surroundings.Excess water formed by the electrochemical reaction, i.e., productwater, is bled into the reservoir 44 by way of the line 42. Thus thereservoir 44 provides a recipient of system product water. A drain spout46 allows releasing excess water to ambient. The heat exchanger willpreferably be controlled by a thermostat 48 which senses the temperatureof the water stream exiting the heat exchanger 40.

In accordance with the invention, the anode flow fields 16' areconnected by a line 50 through a valve 52 to the air inlet line 32, thevalve 52 may be operated in response to a flammability sensor 54 whichwill assure that the amount of hydrogen introduced into the air inletline 32 will remain well below 4%, thereby avoiding a hazardouscondition. The hydrogen, with water vapor, provided to the air inlet 32through the valve 52 is passed into the cathode flow field 16, where itwill react on the cathode catalyst layer 20 (with the oxygen in the air)so as to provide heat and water. The exhaust of the cathode flow field16, illustrated by the arrow 55, will have substantially no hydrogen init, thereby being perfectly safe and non-polluting. In a typical PEMfuel cell, the hydrogen content in the anode exhaust may be 50%-90%. Thefuel purge flow is set such that the volumetric concentration ofhydrogen at the mix point in line 32 is always less than 4% andtypically less than 1% and is therefore quite safe for introduction intothe cathode flow field without any danger. As can be seen by referenceto FIG. 7, fuel cell operation and temperature are hardly affected atall by introducing the anode exhaust into the cathode air stream.

An embodiment of the invention shown in FIG. 2 is identical to that ofFIG. 1, except that a gas composition analyzer 57 monitors the contentof the anode exhaust in the line 50, and controls the valve 52accordingly. An embodiment of the invention shown in FIG. 3 is identicalto that of FIG. 1, except that the valve 52 is controlled by current inthe fuel cell load 60 as indicated by a conventional current detector62. An embodiment of the invention illustrated in FIG. 4 is identical tothat of FIG. 1 except that the valve 52 is controlled by a solenoid 65in response to a clock 66 so as to periodically open and purge the anodeflow field for a predetermined time. Other methodology may be utilizedto control the valve 52. An embodiment of the invention illustrated inFIG. 5 is similar to that of FIG. 1 except that the valve 52 isconnected to the inlet of the blower 30, rather than to its outlet. Themanner of controlling the valve 52 may be in accordance with any of themethodologies mentioned hereinbefore, or otherwise. The point ofapplying the anode exhaust to the cathode inlet air stream, and whetheror how the anode exhaust flow is controlled, are both irrelevant to thepresent invention, which in this form simply provides the anodeeffluent, at least some of the time, to the cathode flow field so as tocombust all remaining fuel, thereby permitting purging of inerts fromthe anode side of the cell without polluting the atmosphere or risking asafety hazard due to fuel in the exhaust.

Various fuel cell operating parameters were measured with and withoutthe anode fuel exhaust recirculated into the cathode process air inletstream. FIG. 7 illustrates the trend of these parameters during thetest. In FIG. 7, testing begins with no fuel added to the cathode; atabout 38 minutes later, the anode fuel exit is recirculated into thecathode process air inlet stream at a rate of 2500 ccm; at about 48minutes, the rate of addition of fuel into the process air inlet streamis reduced to about 650 ccm; and at about 57 minutes, the flow of fuelinto the process air is terminated. The data trends show that theparameters of cell voltage and cell temperature exhibited minimalchanges as a result of injecting fuel into the process air.

Another form of the invention, shown in FIG. 6, does not apply the anodeeffluent to the cathode flow field, but rather applies the anodeeffluent to a catalytic converter 69 via a line 70, the exhaust of which72 is released to ambient. A portion of the cathode exhaust from thefuel cell in a line 74 is diverted to the catalytic converter in a line75 to provide oxidant for the catalytic burning of fuel. The amount offuel which must be consumed is so small that the catalytic converter 69and need not be provided with any special cooling. This form of theinvention may be provided by a separate catalytic converter, disposed ina vehicle remotely of the fuel cell, or it may be provided by a catalystmounted within an anode exhaust manifold, in a fashion similar to themanifold described in commonly owned, copending U.S. patent applicationSer. No. 169,405 filed Oct. 9, 1998, in another context. In any case,another source of air may be provided to the catalytic converter toreact with the fuel which must be combusted.

The aforementioned patent and patent application are incorporated hereinby reference.

Thus, although the invention has been shown and described with respectto exemplary embodiments thereof, it should be understood by thoseskilled in the art that the foregoing and various other changes,omissions and additions may be made therein and thereto, withoutdeparting from the spirit and scope of the invention.

We claim:
 1. A fuel cell system for supplying electric power,comprising:an electrolyte; a cathode catalyst disposed between a cathodesubstrate and said electrolyte on one side of said electrolyte; an anodecatalyst disposed between an anode substrate and said electrolyte on theopposite side of said electrolyte; a cathode flow field having an inletfor flowing an oxidant in contact with said cathode, said cathode flowfield being vented to permit release of exhaust therefrom; an oxidantpump providing oxidant to said cathode flow field inlet; an anode flowfield for allowing a stream of hydrogen to contact said anode, saidanode flow field having an exhaust; a source for providing hydrogen tosaid anode flow field; and an anode flow field exhaust line connectingthe exhaust of said anode flow field to a hydrogen/oxidant catalyticsurface, whereby hydrogen in the exhaust of said anode flow field willbe combusted.
 2. A fuel cell system according to claim 1 wherein:saidexhaust line connects said anode exhaust to said cathode flow fieldinlet, whereby said hydrogen/oxidant catalytic surface comprises thesurface of said cathode substrate of said fuel cell.
 3. A fuel cellsystem according to claim 2 wherein the exhaust of said cathode flowfield is vented to ambient.
 4. A fuel cell system according to claim 1wherein said hydrogen/oxidant catalytic surface comprises a catalyticconverter, separate from said fuel cell, having an oxidant inlet saidanode flow field exhaust line connecting said anode flow field to saidcatalytic converter, the exhaust of said catalytic converter beingvented to ambient.
 5. A fuel cell system according to claim 4 whereinthe exhaust of said cathode flow field is directed to said oxidant inletof said catalytic converter.
 6. A fuel cell system according to claim 1wherein said hydrogen/oxidant catalytic surface comprises a catalyticconverter having an oxidant inlet formed within an anode exit flow fieldmanifold on said fuel cell, said anode flow field exhaust linecomprising said manifold, the exhaust of said catalytic converter beingvented to ambient.
 7. A fuel cell system according to claim 6 whereinthe exhaust of said cathode flow field is directed to said oxidant inletof said catalytic converter.
 8. A fuel cell system according to claim 1wherein said oxidant is air.
 9. A fuel cell system for supplyingelectric power, comprising:an electrolyte; a cathode catalyst disposedbetween a cathode substrate and said electrolyte on one side of saidelectrolyte; an anode catalyst disposed between an anode substrate andsaid electrolyte on the opposite side of said electrolyte; a cathodeflow field, having an oxidant inlet, for flowing oxidant in contact withsaid cathode, said cathode flow field being vented to permit release ofexhaust therefrom; an oxidant pump providing oxidant to said cathodeflow field; an anode flow field for allowing a stream of hydrogen tocontact said anode, said anode flow field having an exhaust; a sourcefor providing hydrogen to said anode flow field; and an anode flow fieldexhaust line connecting the exhaust of said anode flow field to theoxidant inlet of said cathode flow field.
 10. A fuel cell systemaccording to claim 9 further comprising:a flow control valve in saidanode flow field exhaust line.
 11. A fuel cell system according to claim10 wherein said valve is controlled to provide intermittent flow.
 12. Afuel cell system according to claim 11 wherein said valve is controlledto provide intermittent flow in response to a predetermined timeschedule.
 13. A fuel cell system according to claim 10 wherein saidvalve meters flow in response to the amount of hydrogen in the mixtureof the anode exhaust and the inlet oxidant at the inlet to said cathodeflow field.
 14. A fuel cell system according to claim 10 wherein saidvalve meters flow in response to gas composition in said anode flowfield exhaust.
 15. A fuel cell system according to claim 10 wherein saidvalve meters flow in response to current output of said fuel cell.
 16. Afuel cell system according to claim 9 wherein said anode flow field isconnected by said exhaust line downstream of said oxidant pump.
 17. Afuel cell system according to claim 9 wherein said anode flow field isconnected by said exhaust line upstream of said oxidant pump.
 18. A fuelcell system according to claim 9 wherein said oxidant is air.
 19. A fuelcell system according to claim 9 wherein the exhaust of said cathodeflow field is vented to ambient.